Gold nanoparticles-catalyzed activation of 1,2-disilanes: Hydrolysis, silyl protection of alcohols and reduction of tert-benzylic alcohols

Article abstract of DOI:10.1039/c2cc35116a

Gold nanoparticles supported on TiO₂ catalyze under mild conditions the activation of a series of 1,2-disilanes towards hydrolysis and alcoholysis, with concomitant evolution of H₂ gas. For the case of tert-benzyl alcohols, the main or only pathway is reduction to the corresponding alkanes.

Full text of DOI:10.1039/c2cc35116a

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COMMUNICATION
Gold nanoparticles-catalyzed activation of 1,2-disilanes: hydrolysis, silyl
protection of alcohols and reduction of tert-benzylic alcoholsw
Charis Gryparis and Manolis Stratakis*
Received 17th July 2012, Accepted 12th September 2012
DOI: 10.1039/c2cc35116a
Gold nanoparticles supported on TiO2 catalyze under mild
conditions the activation of a series of 1,2-disilanes towards
hydrolysis and alcoholysis, with concomitant evolution of H2
gas. For the case of tert-benzyl alcohols, the main or only
pathway is reduction to the corresponding alkanes.
oxidative hydrolysis occurs (Table 1) yielding the corres-
ponding 1,3-disiloxane, accompanied by evolution of H2 gas.
The reaction is feasible in a variety of solvents, including
DCM, hexane or toluene, yet, it proceeds significantly faster
in ethyl acetate. The overall reaction stoichiometry is such that
the oxygen atom of H2O is formally inserted into the Si–Si
bond of the 1,2-disilane, with formation of one equivalent of
H2. For the case of hexaphenyldisilane (Table 1, entry 5) the
corresponding silanol, was exclusively formed. Au/Al2O3 and
Au/ZnO11 also catalyze the hydrolyses, less efficiently, however
(see ES1w). The reactions are heterogeneous in nature, as after
heating a slurry of Au/TiO2 in ethyl acetate for two hours, the
gold content in the supernatant solution was below ppb level
(ICP analysis).9b Notably, among homogeneous Au(I) catalysts,
Ph3PAuNTf2 catalyses the hydrolytic oxidation of PhMe2SiSi-
Me2Ph (see ESIw), however, at more than two orders of magnitude
slower rate, while [(2-biphenyl)di-tert-butylphosphine]AuSbF6
provides traces of products.
Despite the significant body of gold-catalyzed reactions in
which Au(I) is acting as a soft p Lewis acid,1 little progress has
been achieved regarding the participation of gold in reactions
proceeding through oxidative insertion of the metal into a s
bond. Such reactions are long known to occur readily in the
presence of Pd, Pt and other late transition metals. As Au(I)
has the same electronic configuration to Pd(0), Au(I) sub-
stances should formally expected to be catalytically active in
typical cross coupling reactions catalyzed by palladium. The
few reports so far deal with the homo-coupling of aryl
boronates,2 Suzuki3 or Sonogashira4 cross coupling reactions.
Interestingly, the majority of those cases involve catalysis by
supported gold nanoparticles. The ability of Au(I) to participate
in Sonogashira cross coupling reactions has been disputed, and
ascribed to traces of Pd impurities present in commercially
available gold catalysts.5 However, an indirect proof for the
catalytic efficiency of palladium-free gold was shown immediately
after.6 Recently, evidence was provided that Au(I) can sponta-
neously undergo oxidative insertion into s Si–Si bonds forming
bis(silyl)gold(III) complexes,7 typical behavior of other late transi-
tion metals.8 It was actually questioned7a whether gold-catalyzed
bis-silylation reactions are achievable.
A closer analogy to the above presented Au-catalyzed hydrolytic
oxidation of 1,2-disilanes, is the alcoholysis of hexamethyldisilane in
the presence of PdCl(n3-C3H5)2Á(Ph3P)2 reported by Hayashi.12
Thus, we examined the effectiveness of Au/TiO2 to catalyze
the silylative protection of alcohols with Me3SiSiMe3. The
TMS-silylation of alcohols is traditionally carried out either
by reaction with TMSCl in the presence of a stoichiometric
base, or with hexamethyldisilazane under catalysis conditions.13
Trimethylsilylation of alcohols is also feasible with hexamethyl-
disilane promoted by TBAF.14 We found that a series of
Our continuous interest on catalysis by supported gold
nanoparticles supported on titania (Au/TiO2),9 and especially
the fact that we have observed the facile dehydrogenative bis-
silylation of alkynes by tetramethyldisiloxane,10 a reaction
most likely proceeding via oxidative insertion of gold into
the Si–H bond, urged us to examine whether Au/TiO2 can
activate s 1,2-disilanes towards addition from nucleophiles.
We found that upon adding Au/TiO2 (1 mol%) to a mixture of
a symmetrical 1,2-disilane and 1.2 equiv. of H2O, quantitative
Table 1 Oxidative hydrolysis of 1,2-disilanes catalyzed by Au/TiO2
Entry Substituents
Time/Temperature Yielda (%)
1
2
3
4
5
R1 = R2 = R3 = Me
R1 = R2 = R3 = Et
R1 = R2 = Me, R3 = Ph 30 min/25 1C
R1 = R2 = Ph, R3 = Me 1 h/25 1C
1.5 h/25 1C
3 h/55 1C
>99b
96
98
98c
92d
R1 = R2 = R3 = Ph
5 h/55 1C
Department of Chemistry, University of Crete, Voutes, 71003,
Iraklion, Greece. E-mail: stratakis@chemistry.uoc.gr;
Fax: +30 2810 545001; Tel: +30 2810 545087
w Electronic supplementary information (ESI) available: Spectro-
scopic data for all products (1H, 13C NMR). See DOI: 10.1039/
c2cc35116a
a
b
Isolated yields. Calculated by G. C. using an internal standard.
c
The overall yield represents the total amount of 1,3-disiloxane +
d
silanol in a relative ratio 80/20. Ph3SiOH was exclusively formed
(2 equiv. of H2O).
c
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Table 2 Protection of primary and secondary alcohols as silyl ethers
by symmetrical 1,2-disilanesa catalyzed by Au/TiO2
Table 3 Reaction of hexamethyldisilane with tertiary alcohols cata-
lyzed by Au/TiO2
Alcohol
Silylated product
Yieldb/Time
Alcohol
Alkane (a)
12a (75%)
TMS-alcohol (b)
12b (25%)
Yieldb/Time
82%/1.5 h
1
1
97%/1.5 h
92%/1 h
13a (73%)
14a (60%)
15ac
13b (27%)
14b (40%)
––
81%/1.5 h
94%/2 h
2
3
4
96%/2 h
96%/1 h
97%/3 h
95%/24 h
5
6
93%/1.5 h
87%/16 h
16a (73%)
16b (27%)
96%/1 h
7
8
8
8
9
96%/3 h
93%/3 h
85%/24 h
88%/4 h
98%/4 h
––
––
17bd
18bd
67%/3 h
62%/3 h
a
b
Molar ratio hexamethyldisilane/alcohol = 5/1. Isolated yield.
d
Molar ratio hexamethyldisilane/alcohol = 8/1. The lower yield is
c
due to the partial dehydration of the alcohols.
The participation of Au in the reaction was proven by the fact
that neither disilanes undergo alcoholysis in the absence of
Au/TiO2, nor the support (titania) is catalytically active. In
addition, homogeneous Au(I) catalysts are practically inactive.
Tertiary alcohols react much more slowly, requiring refluxing
conditions to go to completion and excess of hexamethyl-
disilane (Table 3). Also, occasionally minor byproducts from
dehydration of the alcohols were seen in the crude reaction
mixture. Surprisingly, in the case of tertiary benzylic alcohols,
the anticipated TMS-silylation pathway is a minor one, with
the major product being the corresponding alkane. Notably,
in the case of triphenylmethanol, the only product was triphenyl-
methane. To the best of our knowledge, this is the first example of
alcohol ‘‘deoxygenation’’ to an alkane, without any external
hydride added. For instance, benzylic alcohols can be reduced
into alkanes by hydrosilanes in the presence of a Lewis acid.
From the mechanistic point of view, significant information
was gained by the fact that during the progress of the alcoholysis
or hydrolysis of PhMe2SiSiMe2Ph and Ph2MeSiSiMePh2,
PhMe2SiH and Ph2MeSiH, respectively, were seen in varying
amounts within the crude reaction mixture. At the end of the
reaction however they disappear, as they either silylate the
reacting alcohols or hydrolyse through already established
dehydrogenative processes under Au catalysis conditions.16
We propose in Scheme 1, that alcoholysis and hydrolysis
proceed via oxidative insertion of catalytically active Au
species into the s Si–Si bond to form intermediate I, followed
by capture of I by an alcohol or H2O.17 This reaction should
10
11
a
97%/4 h
98%/3 h
1–2 mmol of disilane per 1 mmol of alcohol were used in the case of
open air conditions. If dry solvent and inert atmosphere conditions are
applied, 0.6 mmol of disilane per 1 mmol of alcohol can be used.
b
c
Isolated yield. Molar ratio Me3SiSiMe3/diol = 3/1 (under open air
conditions).
primary and secondary alcohols or phenols are readily and
quantitatively protected as TMS-ethers (Table 2)15 with
hexamethyldisilane under milder conditions to those in the
analogous Pd-catalyzed transformation,12 and more impor-
tantly by using significantly lower loading of catalyst. Ethyl
acetate is the most suitable solvent, whereas under strictly dry
conditions, 50–60% molar ratio of Me3SiSiMe3 is necessary to
drive the reaction to completion, indicative that both trimethyl-
silyl groups of the disilane participate in the reaction. In case
that non-dried EtOAc is utilized (open air conditions), roughly
a 1–2 fold molar excess of Me3SiSiMe3 may be used to
compensate its hydrolytic tendency by the moisture present in
the reaction vessel. As the hydrolysis products of hexamethyl-
disilane are volatile, even though the silylating agent is in excess,
the isolated TMS-protected alcohols are very pure. Silylation of
alcohols is also feasible with bulkier 1,2-disilanes, such as
Et3SiSiEt3, PhMe2SiSiMe2Ph and Ph2MeSiSiMePh2 (Table 2).
c
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Scheme 1 Postulated mechanism regarding the silylative protection
or reduction of alcohols by 1,2-disilanes, catalyzed by Au/TiO2.
form a silyl-protected alcohol or silanol, respectively, and silyl
Au hydrides (II). R3SiAuH18 may either reversibly form a
hydrosilane (seen under our reaction conditions), or protect an
additional molecule of alcohol in a dehydrogenative manner.
The above presented rationalization has some similarities to
the proposed mechanism in the Pd-catalyzed alcoholysis of
hexamethyldisilane, where palladium hydrides were invoked.12,19
The role of the support, apart from stabilizing the catalytic sites,
is not profound. We postulate that the differences in the reaction
rates among TiO2, Al2O3 and ZnO might reflect their affinity16g
to the substrates. It is likely that disilane activation by Au
nanoparticles is driven by ionic gold clusters20 bearing a cationic
Au(I) catalytic site. Recently it was shown that although mono-
nuclear gold(I) complexes are unreactive in Sonogashira cross
coupling reactions, trinuclear cationic gold clusters are highly
active.21 The nature of the active species in processes catalyzed by
Au nanoparticles is obscure, thus we emphasize that our
proposed mechanism is just a reasonable working hypothesis.
Also, as we also observed that in the case of tert-benzylic
alcohols, their trimethylsilyl ethers are not direct precursors of
the alkanes,22 we propose a rational mechanism shown in
Scheme 1 (bottom part), in which the proton of the alcohol
indirectly becomes a reducing hydride. The participation of
intermediate II cannot be excluded.
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Angew. Chem., Int. Ed., 2011, 50, 8320; (b) M. Wilfling and
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Commun., 2011, 47, 803; (c) C. Gryparis, C. Efe, C. Raptis,
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11 Au/TiO2, Au/Al2O3, Au/ZnO (1 wt% in Au) are commercially
available, and have an average gold crystallite size of B2–3 nm.
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Chem., 2010, 12, 94; (b) S. Gharaati, M. Moghadam,
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15 For the general procedure of the silylative protection of alcohols
with hexamethyldisilane see the ESIw.
16 (a) H. Ito, K. Takagi, T. Miyahara and M. Sawamura, Org. Lett.,
2005, 7, 3001; (b) P. Raffa, C. Evangelisti, G. Vitulli and
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In conclusion, we presented herein the first general example of
a palladium resembling, Au-catalyzed activation of 1,2-disilanes,
towards hydrolysis and alcoholysis. The Au/TiO2-catalyzed
alcoholysis of hexamethyldisilane is a simple, atom economical
and clean procedure for the synthesis of trimethylsilyl protected
alcohols. The method is more effective as compared to a known
homogeneous Pd-catalyzed protocol in terms of mildness and
catalyst loading. tert-Benzylic alcohols are primarily reduced by
hexamethyldisilane into the corresponding alkanes, an unpre-
cedented reaction pathway.
17 After this manuscript had been submitted
a stoichiometric
Au-mediated oxygen insertion into a Si–Si bond via a Au(I)/
Au(III) redox sequence was reported, using H2O or O2 as oxidants:
P. Gualco, S. Ladeira, K. Miqueu, A. Amgoune and D. Bourissou,
Organometallics, 2012, 31, 6001. In our work, hydrolysis readily
takes place even under strict inert atmosphere conditions, which
implies that the oxidant is water.
This work was supported by the project ‘‘IRAKLEITOS
II - University of Crete’’ of the Operational Programme for
Education and Lifelong Learning 2007–2013.
18 A. Psyllaki, I. N. Lykakis and M. Stratakis, Tetrahedron, 2012,
68, 8724.
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22 The TMS-ethers of tert-benzyl alcohols (e.g.12b) do not form the
corresponding alkanes upon treatment with excess of hexamethyl-
disilane in the presence of Au/TiO2 even at reflux.
Notes and references
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c
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Chem. Commun., 2012, 48, 10751–10753 10753